Hot-rolled, cold rolled, and plated steel sheet having improved uniform and local ductility at a high strain rate

Patent metadata

Field	Value
Publication number	US-9970073-B2
Application number	US-201013879074-A
Country	US
Kind code	B2
Filing date	Oct 18, 2010
Priority date	Oct 18, 2010
Publication date	May 15, 2018
Grant date	May 15, 2018

Abstract

A multi-phase hot-rolled steel sheet has a metallurgical structure having a main phase of ferrite with an average grain diameter of at most 3.0 μm and a second phase including at least one of martensite, bainite, and austenite. In the surface layer, the average grain diameter of the second phase is at most 2.0 μm, the difference (ΔnH av ) between the average nanohardness of the main phase (nH αav ) and the average nanohardness of the second phase (nH 2nd av ) is 6.0-10.0 GPa, the difference (ΔσnH) of the standard deviation of the nanohardness of the second phase from the standard deviation of the nanohardness of the main phase is at most 1.5 GPa, and in the central portion, the difference (ΔnH av ) between the average nanohardnesses is at least 3.5 GPa to at most 6.0 GPa and the difference (ΔσnH) between the standard deviations of the nanohardnesses is at least 1.5 GPa.

First claim

The invention claimed is:

1. A hot-rolled steel sheet having uniform elongation of at least 23% and local elongation of at least 10% under a dynamic tensile test at a strain rate of 100 s −1 and which comprises a main phase of ferrite and a second phase including at least one of martensite, bainite, and austenite, wherein
in a surface layer of the steel sheet which is a region between the surface of the steel sheet and a location at a depth of 100 μm from the surface, the main phase has an average grain diameter of at most 1.2 μm, the second phase has an average grain diameter of at most 0.7 μm, the difference (ΔnH av ) between the average nanohardness of ferrite (nH αav ) which is the main phase and the average nanohardness of the second phase (nH 2nd av ) is at least 6.0 GPa to at most 10.0 GPa, and the difference (ΔσnH) of the standard deviation of the nanohardness of the second phase from the standard deviation of the nanohardness of the ferrite is at most 1.5 GPa, and
in a central portion of the steel sheet which is a region from a location at a depth of ¼ of the sheet thickness from the surface of the steel sheet to the center of the sheet thickness, the above-described difference (ΔnH av ) in the average nanohardness is at least 3.5 GPa to at most 6.0 GPa and the above-described difference (ΔσnH) in the standard deviation of the nanohardness is at least 1.5 GPa.

2. A cold-rolled steel sheet produced by cold rolling the hot-rolled steel sheet according to claim 1 , having uniform elongation of at least 23% and local elongation of at least 10% under a dynamic tensile test at a strain rate of 100 s −1 and which comprises a main phase of ferrite having an average grain diameter of at most 3.0 μm and a second phase including at least one of martensite, bainite, and austenite, wherein
in a central portion of the steel sheet which is a region from a location at a depth of ¼ of the sheet thickness from the surface of the steel sheet to the center of the sheet thickness, the second phase has an average grain diameter of at most 2.0 μm and an aspect ratio (major axis/minor axis) of greater than 2, the difference (ΔnH av ) between the average nanohardness of ferrite (nH αav ) which is the main phase and the average nanohardness of the second phase (nH 2nd av ) is at least 3.5 GPa to at most 6.0 GPa, and the difference (ΔσnH) of the standard deviation of the nanohardness of the second phase from the standard deviation of the nanohardness of the ferrite is at least 1.5 GPa.

3. A plated steel sheet produced by plating the cold-rolled steel sheet according to claim 2 , having improved uniform elongation of at least 23% and local elongation of at least 10% under a dynamic tensile test at a strain rate of 100 s −1 and which comprises a main phase of ferrite having an average grain diameter of at most 3.0 μm and a second phase including at least one of martensite, bainite, and austenite, wherein
in a central portion of the steel sheet which is a region from a location at a depth of ¼ of the sheet thickness from the surface of the steel sheet to the center of the sheet thickness, the second phase has an average grain diameter of at most 2.0 μm and an aspect ratio (major axis/minor axis) of greater than 2, the difference (ΔnH av ) between the average nanohardness of ferrite (nH αav ) which is the main phase and the average nanohardness of the second phase (nH 2nd av ) is at least 3.5 GPa to at most 6.0 GPa, and

the difference (ΔσnH) of the standard deviation of the nanohardness of the second phase from the standard deviation of the nanohardness of the ferrite is at least 1.5 GPa.

4. A hot-rolled steel sheet as set forth in claim 1 , containing, in mass percent,
C: at least 0.1% to at most 0.2%,
Si: at least 0.1% to at most 0.6%,
Mn: at least 1.0% to at most 3.0%,
Al: at least 0.02% to at most 1.0%,
Cr: at least 0.1% to at most 0.7%, and
N: at least 0.002% to at most 0.015%,
and further containing at least one element selected from
Ti: at least 0.002% to at most 0.02%,
Nb: at least 0.002% to at most 0.02%, and
V: at least 0.01% to at most 0.1%.

5. A cold-rolled steel sheet as set forth in claim 2 , containing, in mass percent,
C: at least 0.1% to at most 0.2%,
Si: at least 0.1% to at most 0.6%,
Mn: at least 1.0% to at most 3.0%,
Al: at least 0.02% to at most 1.0%,
Cr: at least 0.1% to at most 0.7%, and
N: at least 0.002% to at most 0.015%,
and further containing at least one element selected from
Ti: at least 0.002% to at most 0.02%,
Nb: at least 0.002% to at most 0.02%, and
V: at least 0.01% to at most 0.1%.

6. A plated steel sheet as set forth in claim 3 , containing, in mass percent,
C: at least 0.1% to at most 0.2%,
Si: at least 0.1% to at most 0.6%,
Mn: at least 1.0% to at most 3.0%,
Al: at least 0.02% to at most 1.0%,
Cr: at least 0.1% to at most 0.7%, and
N: at least 0.002% to at most 0.015%,
and further containing at least one element selected from
Ti: at least 0.002% to at most 0.02%,
Nb: at least 0.002% to at most 0.02%, and
V: at least 0.01% to at most 0.1%.

Assignees

• Kawano Kaori
• Tanaka Yasuaki
• Tomida Toshiro
• Nippon Steel & Sumitomo Metal Corp

Inventors

• Kawano Kaori
• Tanaka Yasuaki
• Tomida Toshiro

Classifications

• C21D8/0426

  Hot rolling · CPC title

• C21D9/48

  deep-drawing sheets · CPC title

• C21D8/04 Primary

  to produce plates or strips for drawing, e.g. for deep-drawing · CPC title

• C21D8/0463

  following hot rolling · CPC title

• C21D2211/002

  Bainite · CPC title